U.S. patent application number 13/580664 was filed with the patent office on 2012-12-20 for dopant material, semiconductor substrate, solar cell element, and process for production of dopant material.
This patent application is currently assigned to KYOCERA CORPORATION. Invention is credited to Satoshi Kawamura, Youhei Sakai, Mayu Takimoto.
Application Number | 20120318350 13/580664 |
Document ID | / |
Family ID | 44506832 |
Filed Date | 2012-12-20 |
United States Patent
Application |
20120318350 |
Kind Code |
A1 |
Sakai; Youhei ; et
al. |
December 20, 2012 |
DOPANT MATERIAL, SEMICONDUCTOR SUBSTRATE, SOLAR CELL ELEMENT, AND
PROCESS FOR PRODUCTION OF DOPANT MATERIAL
Abstract
A dopant material is disclosed. The dopant material comprises a
polycrystalline silicon and a dopant element in the polycrystalline
silicon. A concentration of the dopant element is at least
1.times.10.sup.18 atoms/cm.sup.3 and no greater than
1.times.10.sup.20 atoms/cm.sup.3. A method for producing a dopant
material is also disclosed. A fused mixture is generated by mixing
and fusing a silicon material with an element that serves as the
dopant source. A coagulate of the dopant material is generated by
cooling and coagulating the fused mixture. A semiconductor
substrate is disclosed. The semiconductor substrate comprises a
semiconductor material to which the dopant material is added. A
solar cell element comprising the semiconductor substrate, a first
electrode, and a second electrode is disclosed. The semiconductor
substrate comprises a first surface and a second surface
corresponding to a rear surface of the first surface.
Inventors: |
Sakai; Youhei; (Yasu-shi,
JP) ; Kawamura; Satoshi; (Higashiomi-shi, JP)
; Takimoto; Mayu; (Omihachiman-shi, JP) |
Assignee: |
KYOCERA CORPORATION
Kyoto-shi, Kyoto
JP
|
Family ID: |
44506832 |
Appl. No.: |
13/580664 |
Filed: |
February 23, 2011 |
PCT Filed: |
February 23, 2011 |
PCT NO: |
PCT/JP2011/053991 |
371 Date: |
August 22, 2012 |
Current U.S.
Class: |
136/256 ; 257/49;
257/E21.14; 257/E29.082; 438/557 |
Current CPC
Class: |
H01L 31/068 20130101;
H01L 31/0288 20130101; C30B 29/06 20130101; C01B 33/02 20130101;
C30B 15/04 20130101; Y02E 10/547 20130101; H01L 31/182 20130101;
H01L 31/0682 20130101; Y02P 70/50 20151101; Y02E 10/546
20130101 |
Class at
Publication: |
136/256 ; 257/49;
438/557; 257/E29.082; 257/E21.14 |
International
Class: |
H01L 29/16 20060101
H01L029/16; H01L 21/22 20060101 H01L021/22; H01L 31/0224 20060101
H01L031/0224 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2010 |
JP |
2010-036839 |
Claims
1. A dopant material, comprising: a polycrystalline silicon; and a
dopant element comprised in the polycrystalline silicon, wherein a
concentration of the dopant element is at least 1.times.10.sup.18
atoms/cm.sup.3 and no greater than 1.times.10.sup.20
atoms/cm.sup.3.
2. The dopant material according to claim 1, wherein the dopant
element is boron.
3. The dopant material according to claim 2, wherein a boron
concentration is at least 5.times.10.sup.18 atoms/cm.sup.3 and no
greater than 5.times.10.sup.19 atoms/cm.sup.3
4. The dopant material according to claim 3, further comprising
oxygen, wherein an oxygen concentration that is measured by
secondary ionization mass spectrometry is at least
1.times.10.sup.16 atoms/cm.sup.3 and no greater than
1.times.10.sup.18 atoms/cm.sup.3.
5. The dopant material according to claim 4, further comprising a
first region positioned upstream in a first direction and a second
region positioned downstream with respect to the first region in
the first direction, wherein the boron concentration in the first
region is greater than the boron concentration in the second
region, and the oxygen concentration in the first region is smaller
than the oxygen concentration in the second region.
6. The dopant material according to claim 5, wherein the oxygen
concentration decreases in a gradual or step-like manner moving
from the second region toward the first region along the first
direction.
7. The dopant material according to claim 6, wherein a reduction
rate of the oxygen concentration in the first region is smaller
than a reduction rate of the oxygen concentration in the second
region.
8. A semiconductor substrate, comprising a semiconductor material
to which the dopant material according to claim 1 is added.
9. A solar cell element comprising: the semiconductor substrate
according to the claim 8, wherein the semiconductor substrate
comprises a first surface and a second surface corresponding to a
rear surface of the first surface; a first electrode that is
positioned on the first surface of the semiconductor substrate; and
a second electrode that is positioned on the second surface of the
semiconductor substrate.
10. A solar cell element comprising: the semiconductor substrate
according to the claim 8, that wherein the semiconductor substrate
comprises a first surface and a second surface corresponding to a
rear surface of the first surface; and a first electrode and a
second electrode that are positioned on the second surface of the
semiconductor substrate and that output to the outside mutually
different electrical charges.
11. The method for producing a dopant material according to claim 1
comprising: generating a fused mixture by mixing and fusing a
silicon material with an element that serves as the dopant source;
and generating a coagulate of the dopant material that comprises
the element that serves as the dopant source and polycrystalline
silicon by cooling and coagulating the fused mixture.
12. The method for producing the dopant material according to claim
11, further comprising coagulating the fused mixture successively
toward one direction by cooling.
13. The method for producing the dopant material according to claim
12 further comprising for cutting the coagulate along a direction
that is perpendicular to the one direction, and pulverizing the
coagulate after cutting.
14. The method for producing the dopant material according to claim
11, wherein boron is used as the element that serves as the dopant
source in generating the fused mixture.
Description
FIELD OF ART
[0001] The present invention relates to a dopant material used for
making a silicon ingot.
BACKGROUND ART
[0002] Conventionally, a silicon substrate has been used as a type
of semiconductor substrate for forming a solar cell element. Such a
silicon substrate is obtained by processing a single-crystal
silicon ingot or polycrystalline silicon ingot produced by the CZ
method or casting method or the like to a prescribed size.
[0003] In order to have the desired electrical characteristics, the
silicon substrate comprises a prescribed amount of a dopant. When
fabricating a p-type semiconductor, boron is generally used as the
element that serves as the dopant source. The following method is
used to produce silicon (a silicon ingot) that comprises dopant of
a prescribed concentration.
[0004] First, a dopant material made of boron alone or of
single-crystal silicon comprising a large amount of boron is
introduced into the silicon material that will serve as the raw
material, and melting by heat is done to produce a fused mixture.
Then, a prescribed method is used to cause coagulation and cooling
of the fused mixture. By doing this, silicon (a silicon ingot)
comprising the prescribed concentration of dopant is produced.
[0005] For example, Japanese Laid-open Patent Publication No.
2006-273668 discloses a silicon substrate for use into a solar cell
element, in which a prescribed amount of a dopant material is
introduced, so that the resistivity is 0.1 .OMEGA.cm to 10
.OMEGA.cm.
[0006] If boron alone is used as the dopant material, in the case
of fabricating a silicon substrate having a large resistivity such
as is used in a solar cell element, an introduced amount of dopant
material becomes very small, and there are cases in which control
thereof is difficult.
[0007] Given this, by using a dopant material made of
single-crystal silicon that comprises a large amount of boron, it
is possible to make the introduced amount of dopant material
introduced large, thereby facilitating control of the dopant
material and, by extension, the dopant concentration. This type of
dopant material is usually used by crushing a single-crystal
silicon ingot comprising a large amount of the dopant source.
However, because single-crystal silicon is extremely hard, it is
difficult to crush.
SUMMARY OF THE INVENTION
[0008] One object of the present invention is to provide a dopant
material having high productivity, a silicon substrate manufactured
using the dopant material, solar cell element, and a method for
producing the dopant material.
[0009] A dopant material according to one embodiment of the present
invention is a dopant material that is to be added to a
silicon-containing semiconductor material, and that comprises an
element that serves as an n-type or a p-type dopant source and
polycrystalline silicon. The concentration of the element serving
as the dopant source is at least 1.times.10.sup.18 atoms/cm.sup.3
and no greater than 1.times.10.sup.20 atoms/cm.sup.3.
[0010] A semiconductor substrate according to an embodiment of the
present invention comprises a semiconductor material to which the
above-noted dopant material is added.
[0011] A solar cell element according to an embodiment of the
present invention comprises the above-noted semiconductor
substrate; a first electrode that is positioned on either a first
surface or a second surface semiconductor substrate, and a second
electrode that is positioned on the second surface of the
semiconductor substrate.
[0012] A method for producing a dopant material according to an
embodiment of the present invention comprises a step of mixing and
fusing a silicon material with an element that serves as the dopant
source, so as to generate a fused mixture, and a step of cooling
the fused mixture, so as to generate a coagulate of the dopant
material that comprises the element that serves as the dopant
source and polycrystalline silicon.
[0013] In the dopant material of this embodiment, by causing
polycrystalline silicon to comprise the element that serves as the
dopant source with a concentration of at least 1.times.10.sup.18
atoms/cm.sup.3 and no greater than 1.times.10.sup.20
atoms/cm.sup.3, compared to a dopant material made of
single-crystal silicon, there are more crystal grain boundaries and
crystal defects. For this reason, it is easy to crush, and it is
possible to reduce the amount of time required to crush the dopant
material finely, thereby facilitating the preparation of the
prescribed amount of dopant material, and improving
productivity.
[0014] In the method for producing a dopant material according to
this embodiment, by adopting the constitution described above, it
is possible to efficiently produce a dopant material in which the
element serving as the dopant source is comprised in
polycrystalline silicon.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic cross-sectional view showing an
apparatus for producing a dopant material (polycrystalline silicon
ingot) according to an embodiment of the present invention.
[0016] FIG. 2 is a descriptive drawing showing an example of the
separation into blocks of coagulates that are obtained as
intermediate products in the process of producing a dopant material
according to an embodiment of the present invention.
[0017] FIG. 3 is a cross-sectional view showing a solar cell
element according to an embodiment of the present invention.
[0018] FIG. 4 is a cross-sectional view showing a solar cell
element according to another embodiment of the present
invention.
EMBODIMENTS FOR PRACTICING THE INVENTION
[0019] A dopant material, a semiconductor substrate, a solar cell
element, and a method for producing the dopant material according
to embodiments of the present invention will be described below,
using drawings.
<Dopant Material>
[0020] In a dopant material according to an embodiment of the
present invention, an element that serves as a dopant source is
comprised in polycrystalline silicon. The purity of the
polycrystalline silicon used can be the same purity as the silicon
material used when producing a silicon ingot for a solar cell, for
example, 99.9999% or greater.
[0021] In this case, the concentration of the element in the dopant
material is at least 1.times.10.sup.18 atoms/cm.sup.3 and no
greater than 1.times.10.sup.20 atoms/cm.sup.3.
[0022] The element that serves as the dopant source may be, for
example, in the condition that all of the element that serve as the
dopant source are dissolved in the polycrystalline silicon.
Alternatively, in the case in which the element that serves as the
dopant source is saturated in the dopant material, a part of the
element that serves as the dopant source may be in the condition of
being precipitated out at the crystal grain boundaries in the
polycrystalline silicon.
[0023] By virtue of this constitution, the dopant material of the
present embodiment has a large number of crystal grain boundaries
and crystal defects. For this reason, it is easy to crush, and it
is possible to reduce the amount of time required to crush it
finely, thereby facilitating the preparation of a prescribed amount
of dopant material, and improving the productivity.
[0024] In this case, the element that is used as the dopant source
is an element that exhibits p-type or n-type characteristics from
Group III or Group V, such as P, B, Ga, Sb, As or the like.
[0025] The dopant material according to the present embodiment is a
p-type dopant material and comprises boron as the element serving
as the dopant source. The concentration of the boron is at least
1.times.10.sup.18 atoms/cm.sup.3 and no greater than
1.times.10.sup.20 atoms/cm.sup.3. Such a dopant material that
comprises boron is suitable for use in the manufacturing of diverse
semiconductor substrates, such as for a solar cell element.
[0026] The resistivity of a dopant material comprising this
concentration of boron is, for example, at least approximately 1.2
m.OMEGA.cm and no greater than 60 m.OMEGA.cm. By making the
resistivity of the dopant material fall within that range, when
manufacturing a silicon ingot for a solar cell, it is possible to
produce the silicon ingot with the prescribed resistivity with good
control. Here, the concentration of the boron (Cb) can be
approximately calculated by the following equation.
Cb=1/(.rho.b.times.q.times..mu.)
[0027] In the above, .rho.b is the resistivity (in units of
.OMEGA.cm), q is the elementary electric charge
(1.6.times.10.sup.19 (in units of C)), and .mu. is the hole
mobility (in units of cm2/(Vs)). Also, a prescribed value may be
used for the mobility .mu. or, because of the dependency on the
dopant concentration, a value obtained from a conversion table
(Irvin curves) based on ASTM F723-81 may be used for the mobility
.mu..
[0028] Also, the boron concentration may be made at least
5.times.10.sup.18 atoms/cm.sup.3 and no greater than
8.times.10.sup.19 atoms/cm.sup.3. By making the boron concentration
fall within this range, it is possible, while reducing the
variation in the resistivity value, to reduce the mass of dopant
material introduced when producing a silicon ingot for use in a
solar cell. By doing this, in addition to improving the cost
advantage, it is possible to achieve good control of the dopant
concentration.
[0029] The value of resistivity as referred to herein, for example,
as shown in FIG. 2 described later, can be taken as the average
value of a first resistivity value that is measured at a first
surface positioned upstream in a first direction and a second
resistivity value that is measured at a second surface positioned
downstream in the first direction in the dopant material, which is
a block. From this average value, it is possible to calculate the
boron concentration comprised in the dopant material, using the
approximate calculation method described above. The first surface
and second surface as referred to herein can be taken as the
upstream and downstream cross-sections described later that occur
by slicing (cutting of the coagulate) to form each of the
blocks.
[0030] In the present embodiment, oxygen is further comprised as an
impurity. The concentration of the oxygen may be at least
3.times.10.sup.15 atoms/cm.sup.3 and no greater than
1.times.10.sup.18 atoms/cm.sup.3 as measured by SIMS (secondary
ionization mass spectrometry). The oxygen concentration may be at
least 1.times.10.sup.16 atoms/cm.sup.3 and no greater than
1.times.10.sup.18 atoms/cm.sup.3 and further may be at least
1.times.10.sup.16 atoms/cm.sup.3 and no greater than
4.times.10.sup.17 atoms/cm.sup.3.
[0031] For example, in the case of using the CZ method to produce a
dopant material in which single-crystal silicon is caused to
contain boron as the dopant source, a quartz crucible into which
few impurities are mixed during melting is used and it is pulled
upward from the silicon melt within the quartz crucible. For this
reason, the silicon melt captures oxygen component comprised in the
quartz, so that it comprises a large amount of oxygen. When this
occurs, a resistivity value that is higher than that indicated by
the actual boron concentration tends to be measured when the oxygen
concentration is high, because the oxygen behaves as a thermal
donor. In the present embodiment, however, the oxygen concentration
is in the above-noted range. It is thus possible to improve the
accuracy of the boron concentration calculated from the value of
resistivity of the dopant material.
[0032] The SIMS measurement is the method in which an accelerated
and narrowly constricted primary ion beam (oxygen, cesium, or the
like) is caused to strike a sample in a vacuum and, of the
particles that fly off the sample surface by sputtering, secondary
ions are extracted by an electric field and subjected to mass
analysis. Then, a comparison is done with a standard sample to
convert to an absolute concentration, thereby enabling measurement
of the oxygen concentration. For example, the following measurement
conditions can be used to measure the oxygen concentration. [0033]
Apparatus used: Cameca IMS-4f [0034] Primary ion type: Cs+ [0035]
Primary ion accelerating voltage: 14.5 kV [0036] Raster region: 125
.mu.m [0037] Analysis region: 30 .mu.m diameter [0038] Measurement
vacuum: 1.times.10.sup.-7 Pa
[0039] The measurement conditions are not restricted to these
conditions.
[0040] In this present embodiment, the dopant material may have a
first region positioned upstream in a first direction and a second
region positioned downstream with respect to the first region. In
this case, the boron concentration in the first region is greater
than the boron concentration in the second region. Additionally,
the oxygen concentration in the first region is smaller than the
oxygen concentration in the second region. By virtue of such a
constitution, by making the oxygen concentration low in a region in
which the dopant concentration is high, it is possible to reduce
the reduction in the measurement accuracy of the resistivity value.
As a result, there is an improvement in the productivity. Also, in
particular if the dopant concentration in the dopant material is
high, because the mass of the dopant material used when producing
the silicon ingot is reduced, the error in the resistivity value
has a great influence on the resistivity value of the silicon
ingot, which is the ultimate product. In contrast, with a dopant
material having the above-noted constitution, it is possible to
easily obtain the desired value of resistivity.
[0041] Additionally, in the present embodiment, the oxygen
concentration may be made to decrease in a gradual or step-like
manner moving from the second region toward the first region along
the first direction. By virtue of such a constitution, along with
the gradually increasing dopant concentration moving toward the
first region, the oxygen concentration is, in reverse, reduced in a
gradual or step-like manner going. By doing this, it is possible to
reduce the reduction in the accuracy of measuring the resistivity
value. Also, because there is no necessity for special processing
to reduce the oxygen concentration in the second region, the result
is that there is a further improvement in the productivity.
[0042] Further, the reduction rate of the oxygen concentration in
the first region may be smaller than the reduction rate of the
oxygen concentration in the second region. By virtue of such a
constitution, by making the reduction rate in the first region
small, that is, by making the reduction rate in the second region
large, it is possible to make the oxygen concentration in the
overall first region low. By doing this, it is possible to further
reduce the reduction in the accuracy of measuring the resistivity
value. As a result, there is a further improvement in the
productivity.
[0043] In the present embodiment, the first region and the second
region may be regions that satisfy the above-described positional
relationship in the first direction and also satisfy the
above-described magnitude relationship between the concentrations
of elements.
[0044] Also, in the present embodiment, the concentration of each
element in the first region can be made, for example, the
concentration of each element measured at the end face of a block
that is positioned upstream in the first direction and also
perpendicular to the first direction. In the same manner, the
concentration of each element in the second region can be made, for
example, the concentration of each element measured at the end face
of a block that is positioned downstream in the first direction and
also perpendicular to the first direction.
[0045] Additionally, the reduction rate of the oxygen concentration
can be established, for example, as follows, if in the first
region. Specifically, if the oxygen concentration on the upstream
side in the first direction in the first region is C1 and the
oxygen concentration on the downstream side in the first direction
is C2, the reduction rate of the oxygen concentration is expressed
as (C2-C1)/C2.times.100%.
[0046] Also, in the present embodiment, although the method of
measuring the concentration of the dopant element has been
described by the example of the dopant material in the form of a
block, the shape of the dopant material is not particularly
restricted to this. For example, it may be, for example, spherical.
That is, as long as the "dopant material" as used herein comprises
an element that will serve as the dopant source with the
above-described high concentration, and can be added to a
semiconductor material, it may be any shape. Also, from the
standpoint of ease of working, the dopant material can be made into
the form of a block or a plate-like body.
<Method for Producing a Dopant Material>
[0047] Next, an embodiment of a method for producing a dopant
material according to the present invention will be described.
First, the manufacturing apparatus used when producing the dopant
material according to the present embodiment will be described.
[0048] The apparatus 21 that produces the dopant material, as shown
in FIG. 1, has a crucible 1, a mold 2, a crucible heating means 3,
mold-releasing material 4, a mold-heating means 5, a cooling means
6, and thermal insulation material 7. A manufacturing apparatus for
a polycrystalline silicon ingot manufactured by casting method can
be used as the manufacturing apparatus 21.
[0049] The crucible 1 comprises a melting part 1a, a holding part
1b, and a pouring spout 1c. The melting part 1a has an aperture
that opens upwardly. The melting part 1a holds within it the
silicon material and the element that will serve as the dopant
source that have been place therein. The materials that have been
placed therein are melted by heating, a silicon melt, which is a
fused mixture, being formed. The silicon melt is then poured into
the mold 2. Quartz of high purity or the like can be used as the
material for the melting part 1a. The holding part 1b holds the
melting part 1a, and is made of, for example, graphite. The pour
spout 1c functions to pour the silicon melt, and is formed at an
upper edge of the melting part 1a. The silicon melt is poured from
the pouring spout 1c into the mold 2. The shapes of the melting
part 1a and the holding part 1b are not restricted to those shown
in FIG. 1.
[0050] The crucible heating means 3 is disposed at the top part of
the melting part 1a. A resistance-type heater or induction heating
coil or the like can be used as the crucible heating means 3.
[0051] The mold 2 has an aperture that opens upwardly. The mold 2
receives the silicon melt formed within the crucible 1 from this
aperture. The mold 2 functions to hold the silicon melt therewithin
while causing coagulation thereof in one direction, from bottom to
top. The mold 2 is made of, for example, a carbon material such as
graphite, or of quartz or fused silica.
[0052] The mold releasing material 4 is coated on the inside
surface part of the mold 2. The mold releasing material 4 may be
formed by coating the mold 2 with a slurry which is obtained by
mixing and agitating silicon nitride into a solution constituted by
an organic binder and a solvent. When this is done, polyvinyl
alcohol, polyvinyl butyral, or methylcellulose or the like can be
used as the organic binder.
[0053] The mold heating means 5 is disposed over the mold 2, and
can be a resistive-type heater or induction heating coil or the
like. The mold heating means 5, by heating the silicon melt poured
into the mold 2 to an appropriate degree, heats the surface of the
silicon melt to an appropriate degree. By doing this, it is
possible to more accurately control the temperature gradient from
the bottom to the top in the silicon melt within the mold 2.
[0054] The cooling means 6 is disposed below the mold 2 and
functions to cause the poured silicon to cool and to coagulate by
removing heat from the bottom thereof. The cooling means 6 is, for
example, made from a metal plate. Specifically, a cooling means
with a structure in which water or a gas is caused to circulate
within hollow metal plates or the like can be used. By the cooling
means 6 approaching the bottom part of the mold 2 in a
non-contacting condition, or coming into contact therewith, the
silicon melt can be cooled from beneath.
[0055] The thermal insulation material 7 is disposed at the
periphery of the mold 2 and functions to suppress the removal of
heat from the sides of the mold. Considering heat resistance and
heat insulation, for example, carbon felt or the like can be used
as the material of the heat insulation material 7.
[0056] The manufacturing apparatus 21 can be disposed within a
vacuum vessel (not shown) and be used under the condition of a
reduction atmosphere such as an inert gas, in which case it is
possible to reduce the intrusion of impurities and oxidation of
materials during the manufacturing process.
[0057] A method for producing a dopant material that uses the
above-described manufacturing apparatus 21 will be described.
[0058] First, the fused mixture is produced. Specifically, a
prescribed amount of silicon material within the crucible 1 is
mixed with an element that will serve as the dopant source. When
this is done, for example, the silicon material is held at the
bottom of the melting part 1a, the dopant source is held thereover,
and silicon material is further held thereover. The dopant source
is held in the region of 15% to 85% of the overall height of the
melting part 1a in the height direction of the melting part 1a.
While the problem of the effect of the inert gas causing the dopant
source to fly up so that the prescribed amount of dopant source is
not dissolved in the silicon melt is reduced, doing this
facilitates the dissolution of the dopant source into the silicon
melt. Then, the crucible heating means 3 melts the silicon material
and the boron, so as to form a fused mixture, that is, a silicon
melt that comprises boron. In the present embodiment, boron is used
as the element that serves as the dopant source. The quantities of
the silicon material and the boron can be, for example,
approximately 5 to 20 g of boron with respect to 100 kg of silicon
material. In this case, it is possible to use a silicon material or
the like that is used when producing a silicon ingot for a solar
cell element, which is polycrystalline silicon.
[0059] Next, the fused mixture that is adjusted to a prescribed
temperature is poured into the pre-heated mold 2. When this is
done, for example, the crucible 1 and the mold 2 can be moved to a
prescribed region, and the fused mixture can be poured from the
crucible 1 into the mold 2.
[0060] Next, the fused mixture is cooled, so as to produce a
coagulate 8. Specifically, as the mold heating means 5 heats the
fused mixture from above, the cooling means 6 cools it from below.
By doing this, a positive temperature gradient is established from
the bottom part of the mold 2 up to the top part, and the fused
mixture is cooled to form a coagulate successively from the bottom
part upward toward the top part. By being subjected to this type of
cooling process, a coagulate 8 that comprises polycrystalline
silicon is produced. After that, several millimeters are removed
from the edge surfaces of the bottom, top, and side parts of the
coagulate 8, which have a high concentration of impurities such as
iron.
[0061] Because boron has a segregation coefficient of 0.8 in
silicon, it becomes concentrated in the silicon melt as the
coagulation progresses. Because of this, the concentration of boron
increases from the bottom part of the coagulate 8 toward the top
part thereof. That is, the resistivity of the coagulate 8 decreases
from the bottom part toward the top part. For this reason, a
plurality of blocks 9 are formed by cutting the coagulate 8 along a
direction that is perpendicular to one direction (the coagulation
direction S), as shown in FIG. 2. The cutting positions can be
appropriately selected so that the difference in resistivity
between the bottom part and the top part of a block 9 is
approximately 1 to 3 m.OMEGA.cm. Although, for example, in the
present embodiment the cutting is done into three blocks, cutting
may be done so as to form three or more blocks.
[0062] As described above, in the coagulate 8, a distribution of
resistivity occurs in one direction (the coagulation direction S),
and the declining gradient of resistivity value tends to be larger
at the top part than at the bottom part. For this reason, as shown
in FIG. 2, the heights (lengths in the coagulation direction S) of
the blocks 9 that are formed can be made so that a block 9
extracted from the bottom part is larger than a block 9 extracted
from the top part. Specifically, the cutting positions can be
determined as the result of calculating the boron concentration
(resistivity) in the coagulation direction S, based on the mass and
the segregation coefficient of the mixed boron. The method used for
calculating the boron concentration (resistivity) can be the
above-described method or the like.
[0063] The above-noted first direction corresponds to the
coagulation direction S in this case, the above-noted upstream side
in the first direction corresponds to the top part in the
coagulation direction S in this case, and the above-noted
downstream side in the first direction corresponds to the bottom
part in the coagulation direction S in this case.
[0064] According to the method of the present embodiment, the
blocks 9 having a plurality of steps of boron concentration are
formed. The obtained blocks 9 can be used as dopant material.
Specifically, when fabricating a silicon ingot, the block 9 can be
sliced or crushed to adjust to the amount of dopant material to be
introduced in accordance with the desired boron concentration.
[0065] In order to obtain the desired semiconductor substrate, the
dopant material is required to have a level of quality that enables
highly precise control of the concentration of the dopant source
element. Given this, the resistivity at 10 to 40 points each on the
top part and bottom part of the obtained block 9 may be, for
example, measured, and the average value thereof can be taken as
the resistivity value of the block 9, so as to control the
concentration of the dopant element in the block 9 to be used as
the dopant material. In this resistivity measurement as well, it is
possible to use the various above-described measurement methods.
For example, in the case of using the non-contact eddy current
decay method, it is possible to increase the accuracy of quality
control of the dopant material because it is possible to reduce the
problem of variation of the resistivity value caused by the
influence of crystal grain boundaries,.
[0066] As described above, the block 9, in which the concentration
of the element serving as the dopant source is controlled based on
resistivity measurements in this manner, is crushed, and the
rubble, sand, particles, or powder, which is obtained by
pulverizing, is used as the dopant material of the prescribed
amount. Dopant material produced by an embodiment such as this
comprises polycrystalline silicon as a main component. That being
the case, it is easy to crush, enabling efficient crushing into
small pieces because the dopant material produced by the present
embodiment comprises a larger amount of crystal grain boundaries
and crystal defects compared with a dopant material that comprises
single-crystal silicon. As a result, it is easy to prepare the
prescribed amount of dopant material, enabling an improvement in
productivity.
[0067] According to the present embodiment, unidirectional
coagulation in the obtained dopant material makes the resistivity
value substantially same in a direction that is perpendicular to
one direction (the coagulation direction S, that is, the first
direction). For this reason, by making the size of the coagulate
be, for example, 300 mm square or larger, it is possible to obtain
a large amount of dopant material having a stable resistivity
value, thereby enabling a reduction of the cost of producing the
dopant material. By using, for example, casting method as the
unidirectional coagulation method, it is easy to obtain a dopant
material that has a large dimension in the lateral direction, that
is, the direction that is perpendicular to the coagulation
direction S. Also, for example, by cutting the coagulate 8 into
squares of 10 to 15 cm along a direction parallel to the
coagulation direction S, it is possible to make the difference in
resistivity over the surface small and also possible to facilitate
use as a dopant material and to improve the productivity
thereof.
[0068] In the manufacturing method of the present embodiment,
crystal seeds occur randomly at locations that are both the most
cooled locations at the bottom part and locations in contact with
the silicon melt, and multiple crystals proceed to grow from these
points as origins. For this reason, the obtained dopant is
generated as polycrystalline silicon in which the orientations of
the individual crystals differ.
[0069] If the mold-releasing material 4 comprises silicon nitride,
not only is mold removal easy, but also it is possible to reduce
the intrusion of oxygen into the silicon melt even when using a
mold 1 that has a silicon oxide component. In this case, in order
to improve the strength of the mold-releasing material 4, a mixture
of silicon nitride and a silicon oxide may be used as the
mold-releasing material 4. If this is done, by making the ratio of
silicon nitride to silicon oxide be 10:0 to 6:4, the increased
amount of silicon oxide enables a reduction in the amount of oxygen
that intrudes into the silicon melt within the mold 1.
[0070] The oxygen concentration in the coagulate 8 decreases from
the bottom part of the coagulate 8 toward the top part thereof.
That is, the oxygen concentration in the coagulate 8 decrease from
the downstream side toward the upstream side along the coagulation
direction S (first direction). This is because of release of
silicon oxide (SiO2) gas from the silicon melt during the cooling
step, which reduces the amount of oxygen that intrudes into the
silicon melt due to the mold-releasing material 4. When this
occurs, the oxygen concentration in the coagulate 8 decreases
exponentially as the coagulation progresses. For this reason, it is
possible to make the oxygen concentration at least
1.times.10.sup.16 atoms/cm.sup.3 and no greater than
4.times.10.sup.17 atoms/cm.sup.3 as measured by SIMS. By making the
oxygen concentration fall within this range, it is possible to
improve the accuracy of calculating the boron concentration from
the value of resistivity.
[0071] It is therefore possible to reduce variation in resistivity
over the surface, which is caused by a non-uniform oxygen
concentration distribution, that is, the thermal donor distribution
over the surface, which is viewed in the form of a dopant material
that includes a single-crystal silicon ingot, when the
polycrystalline silicon comprising dopant material is obtained from
the above-described manufacturing method. That is, in the coagulate
8 produced by the above-described manufacturing method, because the
oxygen concentration is low and it is possible to perform control
so as to achieve uniformity of the resistivity value over the
surface by unidirectional coagulation, it is possible to
manufacture a uniform dopant material. The expression over the
surface as used herein means over a surface that is perpendicular
to the coagulation direction S.
[0072] In the manufacturing method according to the present
embodiment, although the description has been for the case of
pouring the silicon melt produced by melting the silicon material
in the crucible 1 into the mold 2, the silicon material may be
melted within the mold 2.
<Semiconductor Substrate>
[0073] Next, a semiconductor substrate according to the present
embodiment will be described.
[0074] The semiconductor substrate according to the present
embodiment is obtained by manufacturing a semiconductor ingot, that
is, a silicon ingot in which a dopant material produced as
described above is added to a semiconductor material. More
specifically, the semiconductor substrate for use in a solar cell
element and that is obtained by the above-described dopant material
will be described in detail for the present embodiment. In the
present embodiment, silicon is used as the semiconductor material
to which the dopant material is added.
[0075] A polycrystalline silicon ingot for use in a solar cell
element can be manufactured by a variety of known silicon ingot
manufacturing apparatuses. Manufacturing is possible using, for
example, the polycrystalline silicon ingot manufacturing apparatus
having the constitution shown by the schematic cross-sectional view
of FIG. 1. The dopant material produced by the above-described
manufacturing method and the silicon material are placed into the
crucible 1 and heated to melting, and the silicon melt that is
formed is poured into the mold 2, the inside surface of which is
covered by the mold-releasing material 4. Then, the silicon melt is
heated from above by the mold heating means 5 and cooled from the
bottom part by the cooling means 6, so as to cause gradual
unidirectional coagulation from the bottom part side of the mold 2.
By the silicon melt completely coagulating, a polycrystalline
silicon ingot is obtained. When this is done, the amount of dopant
material introduced can be appropriately adjusted in accordance
with the resistivity value of the dopant material, so that the
polycrystalline silicon ingot has the desired resistivity. For
example, 50 to 300 g of dopant material having a resistivity value
of at least 1.2 m.OMEGA.cm and no greater than 60 m.OMEGA.cm may be
introduced with respect to 100 kg of silicon material.
Alternatively, the dopant material and silicon material may be
introduced into the mold 2, not the crucible 1, and melted.
[0076] The silicon substrate (semiconductor substrate) for used in
a solar cell element is obtained by removing the polycrystalline
silicon ingot obtained as noted above from the mold 2, cutting it
to a prescribed size, and then slicing it using a multi-wire saw or
the like.
<Solar Cell Element>
[0077] Next, a solar cell element 10 according to the first
embodiment of the present invention that uses the above-described
semiconductor substrate will be described.
[0078] The solar cell element 10 according to the present
embodiment comprises a semiconductor substrate 11, a diffusion
layer 12, an anti-reflection film 13, a first electrode 14, and a
second electrode 15. It preferably further comprises a BSF (back
surface field) layer 16.
[0079] As shown in FIG. 3, in the semiconductor substrate 11, a
first surface (light-receiving surface) 11a side has a
concavo-convex shape 11b.
[0080] The diffusion layer 12 is formed by diffusing an n-type
impurity to a prescribed depth from the outside surface of the
first surface 11a that has the concavo-convex shape 11b in the
semiconductor substrate 11. By doing this, a p-n junction is formed
between the semiconductor substrate 11 and the diffusion layer
12.
[0081] The anti-reflection film 13 is formed over the surface of
the diffusion layer 12 and is made of, for example, silicon oxide,
silicon nitride, or titanium oxide or the like.
[0082] The first electrode 14 and the second electrode 15 are
formed by coating each first surface 11a and second surface 11c
that corresponds to the rear surface of the first surface 11a of
the semiconductor substrate 11 with a prescribed pattern of an
electrode paste having silver as a main component and then firing.
By the fire-through method, for example, the first electrode 14 can
easily contact the diffusion layer 12. Alternatively, the second
electrode 15 may have an aluminum electrode 15a and a silver
electrode 15b, which are formed by coating the second surface 11c
of the semiconductor substrate 11 with, for example, prescribed
patterns of electrode pastes having aluminum as a main component
and having silver as a main component respectively, and then
firing.
[0083] The solar cell element 10 may further have a BSF layer 16.
The BSF layer 16 is a high-concentration p-type diffusion layer
that is provided on the second surface 11 c side of the
semiconductor substrate 11. In the case in which the BSF layer 16
is formed using aluminum, it is formed by the diffusion of the
aluminum into the semiconductor substrate 11 in the step of coating
and firing an aluminum paste.
[0084] Next, a solar cell element 20 according to the second
embodiment of the present invention will be described. As shown in
FIG. 4, the solar cell element 20 of the present embodiment differs
from the solar cell element 10 according to the first embodiment
with regard to the provision therein of both the first electrode 14
and the second electrode 15 on the second surface 11c side (rear
side) of the semiconductor substrate 11.
[0085] Specifically, in the present embodiment, an n-type diffusion
layer 12 is formed on a part of the second surface 11c, the first
electrode 14 is formed on the n-type region (diffusion region 12)
on the second surface 11c, and the second electrode 15 is formed on
the p-type region (BSF layer 16) on the second surface 11c. In this
manner, the first electrode 14 and the second electrode 15, which
output to the outside mutually different electrical charges, are
provided on the second surface 11c of the semiconductor substrate
11. When this is done, for example, both the first electrode 14 and
the second electrode 15 may be comb-tooth shaped and also provided
so that there is a space therebetween.
[0086] While the foregoing has been a description of various
embodiments of the present invention, the present invention is not
restricted to the above-noted embodiments, and can be subjected to
many modifications and changes within the scope of the present
invention. That is, the present invention encompasses, of course,
various combinations of the above-noted embodiments.
[0087] For example, the silicon material used to produce the dopant
material may be the bottom part material or end part material of a
silicon ingot for a solar cell element manufactured by casting
method that cannot be used as a silicon substrate. If this is done,
the bottom part material or end part material is material that is
obtained by blasting or grinding to remove approximately 0.4 to 5
mm of the layer of the surface that had made contact with the
mold-releasing material. By doing this, metal impurities and the
mold-releasing material are removed, enabling re-use as silicon
material.
DESCRIPTION OF THE REFERENCE SYMBOLS
[0088] 1 Crucible [0089] 1a Melting part [0090] 1b Holding part
[0091] 1c Pouring spout [0092] 2 Mold [0093] 3 Crucible heating
means [0094] 4 Mold-releasing material [0095] 5 Mold heating means
[0096] 6 Cooling means [0097] 7 Thermal insulation material [0098]
10, 20 Solar cell element [0099] 11 Semiconductor substrate [0100]
11a First surface [0101] 11b concavo-convex surface [0102] 11c
Second surface [0103] 12 Diffusion layer [0104] 13 Anti-reflection
film [0105] 14 First electrode [0106] 15 Second electrode [0107] 16
BSF layer [0108] 21 Dopant material manufacturing apparatus
* * * * *